The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, HDDs have been desired to store more information in its limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective, which in turn typically requires the design of smaller and smaller components.
As a technique for narrowing the spacing between elements of a magnetic head down to a few nanometers, thermal fly-height control (TFC) has been used where part of the media-facing surface of a read or write element is deliberately made to project due to thermal expansion caused by a heating element. This increases the risk of contact between the read or write element and the magnetic disk surface, so magnetic, thermal, and mechanical stress tend to be increased compared with conventional magnetic heads. To deal with this, improving stabilization of the magnetic head is beneficial.
In this situation, if some kind of stress acts on the head during operation of the HDD, this may result in read errors. There are various causes of such stress, one of them being changes in the magnetization condition of a hard bias film of the magnetic head, which is part of a read element. The read element in the magnetic head constantly continues to receive magnetic stress from the media and in addition, when a collision with the media occurs, magnetic stress becomes even larger. It is surmised that the direction of magnetization of the hard bias film may change when such a special event occurs. In particular, it is thought that if the hard bias magnetization changes in a vicinity of the read element, this may influence the reading ability of the read element, and thus be associated with read errors. With the reduction in read element size in recent years, the effect of changes in hard bias magnetization has increased, and can no longer be neglected.
The problems associated with the hard bias magnetization changes are not manifested uniformly and it is believed that areas of local weakness (low coercive force) are randomly distributed in the film, due to variability of the grain size and orientation of the magnetic anisotropy. The direction of magnetization of such areas of local weakness may be changed by magnetic field stress in the HDD. Also, the direction of magnetization of the hard bias film near the read element may easily become directed in a direction perpendicular to the media-facing surface, due to the effect of the demagnetizing field produced in an end portion of the hard bias film.
If there are areas of local weakness near the read element, the effects described above may become pronounced, producing changes in the direction or intensity of the bias magnetic field applied to the read element; unfortunately, this is associated with instability of the magnetic field influencing the read element which may cause read errors. Reducing the average grain size of the hard bias film has been considered as one way to solve this problem, since, if the magnitude of the local change of the hard bias film magnetization is made small in relation to the size of the sensor, the effects on the magnetic field influencing the read element may be reduced. However, this method is difficult to adopt, since this method lowers the coercive force of the grains themselves and so impairs their stability with regard to external magnetic fields.
In contrast, increasing the coercive force by increasing the grain size of the hard bias film itself has also been considered. In this case, even if the grain size is increased, some proportion of small grains will always remain in the vicinity of the read element, due to the manufacturing process of the wafer or mechanical grinding processing of the media-facing surface, which results in local areas of low coercive force not being able to be eliminated. Thus, as described above, the problem of hard bias magnetization stabilization is still unsolved.